1. Field of the Invention
The present invention relates to a wide-angle zoom lens with high variable magnification, and to an imaging apparatus in which the zoom lens is provided.
2. Description of the Related Art
In recent years, wide-angle zoom lenses with high variable magnification were requested for use in consumer-grade video cameras, cameras for motion picture photography in TV broadcasting, surveillance or monitor cameras for business operations, and the like.
As a zoom lens satisfying such a request, a four-group rear-focus-type zoom lens is widely known (please refer to U.S. Pat. No. 7,738,185 (Patent Document 1)). The four-group rear-focus-type zoom lens is composed of four lens groups of a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power, which are arranged in this order from the object side of the zoom lens. Zooming is performed by moving the second lens group. Further, the fourth lens group is moved in such a manner that a fluctuation of the position of an image plane caused by zooming is corrected so that the zoom lens is focused.
However, in all four-group rear-focus-type zoom lenses available as products, the lens types of the first lens groups and the lens types of the second lens groups closely resemble each other. Further, in most of the available four-group rear-focus-type zoom lenses, one or two lenses having positive refractive power and a cemented lens are arranged in the first lens group (please refer to Patent Document 1).
The zoom lens disclosed in Patent Document 1 has high variable magnification of 20 times or higher. However, the maximum angle of view when the zoom lens is set at wide angle end is approximately 60 degrees, which is not particularly large. When the maximum angle of view of the zoom lens adopting such a lens type should be further increased, the diameter of a front lens of the zoom lens needs to be increased to increase the height of off-axial rays entering the first lens group at wide angle end.
However, in an imaging apparatus adopting such a rear-focus-type zoom lens, if the height of off-axial rays (ray or beam) entering the first lens group is increased to achieve a wide angle of view and high variable magnification at the same time, the size of the optical system becomes large. Therefore, substantially no such zoom lens has been manufactured for sale.
Meanwhile, many kinds of wide conversion lens that can increase the angles of view of zoom lenses without increasing the diameters of front lenses of the zoom lenses have been proposed. For example, a two-element wide conversion lens is known (please refer to Japanese Patent No. 3342157 (Patent Document 2)). In the two-element wide conversion lens, a lens having negative refractive power and a lens having positive refractive power are arranged in this order from the object side.
Further, a wide-angle zoom lens in which the wide conversion lens disclosed in Patent Document 2 and a master lens that is a zoom lens on which the wide conversion lens is mountable are integrated with each other has been proposed. Specifically, an optical system corresponding to the wide conversion lens and an optical system corresponding to the first lens group in the master lens are optimally integrated with each other.
As the wide-angle zoom lens, in which the optical system corresponding to the wide conversion lens and the optical system corresponding to the master lens are assumed to be inseparable from each other, for example, a zoom lens having an extremely wide angle of view (maximum angle of view is 88 degrees) at wide angle end is known (please refer to U.S. Pat. No. 7,589,909 (Patent Document 3)).
In the zoom lenses disclosed in Patent Documents 2 and 3, an afocal optical system having a large air space between a lens group having negative refractive power and a lens group having positive refractive power is adopted. Accordingly, the inclination of rays entering lenses arranged after the afocal optical system is reduced, and a wide angle of view is achieved. In the zoom lens disclosed in Patent Document 2, each of the lens group having negative refractive power and the lens group having positive refractive power, which constitute the afocal optical system, is composed of a lens.
In a zoom lens adopting an afocal optical system to achieve a wide angle of view, as described above, the size of a first lens group even alone is extremely large, and the cost of an apparatus increases. Further, when the variable magnification of such a zoom lens increases, both of the size and the cost of the apparatus further increase. Therefore, the variable magnification of a wide-angle zoom lens adopting the afocal optical system normally remains approximately 8 to 10 times.
Further, a zoom lens including a first lens group composed of four lenses, which are a small number of lenses, and which can achieve a maximum angle of view of approximately 70 to 80 degrees is known (please refer to U.S. Pat. No. 6,049,431 (Patent Document 4) and Japanese Unexamined Patent Publication No. 2009-3342 (Patent Document 5) for example). The major characteristic of these wide-angle zoom lenses is that an air space (gap) is provided between a first-group first lens and a first-group second lens. Meanwhile, in many generally-known wide-angle zoom lenses, a first-group first lens and a first-group second lens are cemented together as in the zoom lens disclosed in Patent Document 1.
The wide-angle zoom lenses disclosed in Patent Documents 4 and 5 effectively use air spaces (air lenses) to correct distortion and curvature of field at wide angle end in an excellent manner. It may be considered that the first lens and the second lens in the wide-angle zoom lenses disclosed in Patent Documents 4 and 5 play similar roles to the role of the wide conversion lens disclosed in Patent Document 2. In Patent Documents 4 and 5, the first lens and the second lens are structured in such a manner to prevent the lens diameter of the first lens group from becoming large. As described, zoom lenses composed of small numbers of lenses, and which can achieve wide angles of view without increasing the lens diameters of the first lens groups are also known.
The sizes of the zoom lenses disclosed in Patent Documents 4 and 5 are small, and wide angles of view of approximately 70 to 80 degrees are achievable. However, the variable magnification of this type of zoom lens is approximately 12 times, which is low.
Therefore, there is a request to increase variable magnification of zoom lenses, as disclosed in Patent Documents 4 and 5, which are small in size but can achieve wide angles of view of approximately 70 to 80 degrees, to approximately 20 times.
However, for example, in the zoom lens disclosed in Patent Document 5, which can achieve a wide angle of view of 80 degrees, the first-group first lens has a double-concave shape having a large curvature. If high variable magnification of 20 times is tried to be obtained in such lens structure, it becomes difficult to balance a curvature of field and distortion at wide angle end and a spherical aberration at telephoto end.
Further, one of problems to be solved in increasing variable magnification is correction of longitudinal chromatic aberrations. For example, in the zoom lens disclosed in Patent Document 5, the Abbe number of the material of each lens having positive refractive power in the first lens group is approximately 50 to 60. If the variable magnification is tried to be increased using the same material for these lenses, chromatic aberrations at telephoto end become extremely large.
Such large chromatic aberrations are generated, because when the power of a lens or lenses having negative refractive power and the power of a lens or lenses having positive refractive power are increased to increase the angle of view, if low-refractive-index low-dispersion material is used as the material of the lenses, it becomes necessary to increase the curvatures of the lenses. Consequently, it becomes difficult to correct aberrations of off-axial rays at wide angle end. To avoid such events, the zoom lens disclosed in Patent Document 5 uses a material having a refractive index of 1.7 or higher, which is a high-refractive-index material. However, use of such a material is disadvantageous to correction of chromatic aberrations. As described above, the zoom lens disclosed in Patent Document 5 may be advantageous to increase an angle of view, but disadvantageous to increase the variable magnification, because correction of chromatic aberrations becomes difficult.
As described above, there is a problem that it is difficult to prevent generation of aberrations in a zoom lens that can achieve a wide angle of view and high variable magnification at the same time without increasing the size of an apparatus by preventing an increase in the lens diameter of the first lens group of the zoom lens and an increase in the number of lenses in the zoom lens.